---
title: "Sliding mesh, constant mesh, synchromesh gearboxes"
subject: "Machine Design II"
chapter: 5
key: "semester_7/machine_design_ii:sliding-constant-synchromesh-gearbox"
source: https://www.bctengineer.com/q/semester_7/machine_design_ii/sliding-constant-synchromesh-gearbox
site: BCT Engineering
---

# Sliding mesh, constant mesh, synchromesh gearboxes

Machine Design II · Chapter 5: System Design

- Asked in: year not given, 6 marks
- Video: [Sliding mesh gearbox | How Gearbox works | Construction and Operation | Automobile Engineering](https://www.youtube.com/watch?v=xsulf_yCqSU) — MECH Tech Simulations
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/machine_design_ii/sliding-constant-synchromesh-gearbox
- Whole chapter: https://www.bctengineer.com/md/semester_7/machine_design_ii/chapter-5

## Question

Differentiate between the sliding mesh, constant mesh and synchromesh gearboxes used in automobiles. Explain the working of the synchromesh device.

## Short answer

Sliding mesh: spur gears slide into mesh, noisy, needs double declutching. Constant mesh: all helical gears stay in mesh, dog clutches engage, still needs double declutching. Synchromesh: adds cone synchronizers; the blocker ring friction-cone matches speeds before sleeve locks the gear, allowing silent, easy shifting without double declutching.

## Answer

An automobile gearbox changes the torque-speed ratio between engine and wheels and gives reverse and neutral.

#### Comparison
| Point | Sliding mesh | Constant mesh | Synchromesh |
|---|---|---|---|
| Gear engagement | Gears on the main shaft slide along splines to mesh with countershaft gears | All gears always in mesh; dog clutches slide to lock a free gear to the shaft | Constant mesh plus cone synchronizers that equalise speeds before engagement |
| Gear type | Spur (straight) | Helical (quiet) | Helical |
| Gear change | Needs double declutching and skill; noisy | Easier, but still needs double declutching | Very easy, no double declutching |
| Wear and noise | High; teeth damaged by clashing | Less wear, quieter | Least wear and noise |
| Cost | Cheapest | Moderate | Highest |
| Use | Old cars, some trucks | Heavy vehicles | Modern cars |

#### Working of the synchromesh
```text
 Gear (with cone)  Blocker ring  Hub/sleeve
     |\               |\            |
     | \_cone_______  | friction    | shift
     |  engages      |  cone       | fork
```
1. The driver moves the gear lever; the fork pushes the sleeve toward the selected gear.
2. A **blocker ring** (with a cone surface) is pressed on the cone of the gear. Friction brings the speeds of the shaft hub and the gear to the same value.
3. The blocker ring prevents the sleeve from sliding on until the speeds match, so there is no clash.
4. After speeds are equal, the sleeve slides over the dog teeth of the gear and locks it to the shaft.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How are gears engaged in a sliding mesh gearbox?
   A. Gears are always in mesh with dog clutches
   B. A cone synchronizer matches speeds first
   C. Gears on the main shaft slide along splines to mesh with countershaft gears
   D. Planetary gears lock by bands
   Answer: C. Sliding mesh uses spur gears that move on splines, so clashing and noise are likely. (Key point: Sliding mesh)

2. What is true of a constant mesh gearbox?
   A. All gears are always in mesh and dog clutches lock a free gear to the shaft
   B. Only the first gear is in mesh
   C. It uses no helical gears
   D. It needs no declutching of any kind and has cone rings
   Answer: A. Dog clutches engage gears that are permanently meshed, and double declutching is still needed. (Key point: Constant mesh)

3. What does the blocker ring do in a synchromesh?
   A. It transmits the engine torque
   B. It reverses the gear direction
   C. It locks the gear to the shaft permanently
   D. It stops the sleeve sliding on until friction on the cone has equalised the speeds
   Answer: D. The cone friction brings hub and gear to the same speed, and the ring blocks engagement until then so there is no clash. (Key point: Blocker ring)

4. What happens after the speeds match in a synchromesh?
   A. The sleeve slides over the dog teeth of the gear and locks it to the shaft
   B. The gear is released
   C. The blocker ring rotates the gear
   D. The engine stops
   Answer: A. Engagement of the dog teeth comes last. This is why no double declutching is needed. (Key point: Synchromesh sequence)

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